1. Field of the Invention
The present invention relates to a drive device for a vibration type motor and an image forming apparatus and, more particularly, to a device which is suitably applied to an electrophotography apparatus such as a printer apparatus, copying machine, facsimile apparatus, and the like, which rotate rotary members such as a photosensitive drum, transfer belt, transfer drum, and the like using a vibration type (vibration wave) motor as a drive means with high rotational precision.
2. Related Background Art
A vibration type motor (e.g., a vibration wave motor) normally excites a plurality of vibrations in a vibration member using frequency beyond the audible range, and obtains a driving force by synthesizing these vibrations, as proposed in Japanese Patent Application Laid-Open No. 58-148682, and as for its driving performance, stable rotation performance at constant speed is realized, as described in detail in Japanese Patent Application Laid-Open Nos. 63-1379, 60-176470, 59-204477, and the like.
In the prior art, in an electrophotography apparatus which rotates a photosensitive drum and transfer belt using a vibration wave motor as a drive means with high rotational precision, when a recording paper sheet enters the transfer belt or a paper sheet on the transfer belt enters a fixing device, an abrupt load drift occurs.
At this time, in the case of a vibration wave motor having a normal speed control, when the load in the vibration wave motor increases abruptly, since the vibration wave motor increases its torque in correspondence with the load, the driving frequency to be supplied to the vibration wave motor is controlled so as to abruptly decrease. As a result, the vibration wave motor reaches a range wherein it cannot operate, and stops.
FIG. 11 shows this state. The abscissa plots time, whereas the ordinate plots the driving frequency. FIG. 11 indicates a case wherein the motor reaches a stable speed state from the beginning of its driving. As shown in FIG. 11, the driving frequency drifts up and down at given cycles, but such drifts are caused by the inertia of the load, and the like. Hence, various cases occur depending on the load.
When such a vibration wave motor is used as drive means for driving a photosensitive drum or for driving an endless transfer medium conveyor belt for conveying a transfer medium in a color electrophotography apparatus (in which photosensitive drums of the respective colors are lined up along the convey direction of a transfer medium), and when a transfer medium such as a recording paper sheet becomes attached to the transfer medium conveyor belt or enters a fixing device for fixing a toner image on the transfer medium by heat and pressure while the vibration wave motor is driven in a steady state, as shown in FIG. 12, an abrupt load drift occurs. Hence, the driving frequency changes abruptly depending on the control response characteristics, as indicated by peak xe2x80x9caxe2x80x9d in FIG. 12.
Furthermore, when the load is heavy, peak xe2x80x9cbxe2x80x9d appears, and the driving frequency decreases to a range where the vibration wave motor can no longer rotate.
For this reason, when the motor temporarily comes to a halt, the driving frequency must be returned to an initial startup frequency again to restart the motor, thus requiring special processes. Hence, normal control cannot restart the motor.
The range where the vibration wave motor cannot operate corresponds to a portion where the motor can no longer rotate when the driving frequency decreases, and is called a resonance frequency fc of the vibration wave motor, as shown in FIGS. 13A and 13B. As is known, since the resonance frequency fc varies in units of vibration wave motors, and also varies depending on the motor temperature, if the lower limit value of the driving frequency is set to be a small value to inhibit the driving frequency from dropping below this lower limit value, the resonance frequency itself may assume a value larger than the lower limit value. For this reason, a small lower limit value cannot be set. When a large lower limit value is set, the aforementioned problem can be solved, but a value smaller than this lower limit value may be exhibited due to normal frequency drift (e.g., a slight load drift or frequency drift in constant speed control) in a normal driving state, and the frequency drift range, which should be allowed, is limited.
It is an object of the present invention to provide a drive device for a vibration type motor, which guarantees stable driving by reducing the possibility of the motor coming to a stop even when the driving frequency has varied due to a load drift.
One aspect of the invention is to provide a drive device for a vibration type motor which obtains a driving force by applying a periodic signal to an electro-mechanical energy conversion element portion provided with a vibration member, comprising limit value setting means for setting a lower limit value of a driving frequency, which is determined in accordance with a change in frequency of the periodic signal while the motor is being driven, and inhibition means for inhibiting the frequency of the periodic signal from shifting in a direction to become lower than the lower limit value set by the limit value setting means during driving of the motor.
One aspect of the invention is to provide an image forming apparatus, which has one or a plurality of image carriers, exposure means for forming a latent image by irradiating each of the image carriers with image light, development means for developing the latent image with toner, convey means for conveying a transfer medium toward a transfer position, and transferring a toner image carried on each of the image carriers onto the transfer medium, and one or a plurality of vibration type motors each of which obtains a driving force by applying a periodic signal to an electro-mechanical energy conversion element portion provided with a vibration member, and uses the vibration type motors as drive sources for the one or the plurality of image carriers and the convey means, comprising limit value setting means for setting a lower limit value of a driving frequency, which is determined in accordance with a change in frequency of the periodic signal while the motor is being driven, and inhibition means for inhibiting the frequency of the periodic signal from shifting in a direction to become lower than the lower limit value set by the limit value setting means during driving of the motor.
One aspect of the invention is to provide a drive device for a vibration wave motor, which controls driving by applying a driving periodic signal to the vibration wave motor, having a measurement mode for detecting the temperature of the vibration wave motor and the characteristic value of the driving periodic signal, wherein a critical frequency is set on the basis of the characteristic value and temperature upon operating the motor measured in this mode.